Hydraulically-actuated variable valve timing mechanisms have been well known. Such mechanisms include an advanced angle chamber and a retarded angle chamber defined by a housing, which rotates in conjunction with the rotation of the crankshaft of the engine, and a vane body, which rotates integrally with a camshaft. Oil pressure is applied to the advanced angle chamber and the retarded angle chamber to change a phase angle of the camshaft relative to the crankshaft, thereby changing the opening/closing timing of the valve.
Patent Document 1 discloses a hydraulically-actuated variable valve timing mechanism, which is provided with a locking mechanism that locks the operation of the variable valve timing mechanism. The locking mechanism has a stopper pin that fixes the vane body at a predetermined rotation angle relative to the housing (i.e., a locking pin that fixes the phase angle of the camshaft relative to the crankshaft). In releasing the stopper pin from a locking state by using oil pressure and transiting to a phase control, oil pressures before and after it is controlled by a hydraulic control valve, which adjusts the oil pressure to be applied to the advanced angle chamber and the retarded angle chamber, are calculated to avoid unsuccessful release of the stopper pin from the locking state due to variations in the pressure applied to the advanced angle chamber and the retarded angle chamber during release of the stopper pin from the locking state, by adjusting the timing of transition to the phase control according to the obtained oil pressures before and after the valve control.